Is there any way I can create an edge across these two points?
You can do this with the knife tool.


Select the upper part of the object and hide it for now, so it's not blocking the cut.
Go in Orthographic top view Numpad 7
Activate the knife tool K select the vertex at the corner you marked with the 90 degree red angle.
Drag the cut line and press C to activate "angle constraint", making it easy to snap the cut to the y axis.
drag the cut over the last edge you want to cut an confirm the cut with LMB followed by Enter.


You should have something similar to this little oversimplified visual example:



Don't forget to unhide the previously hidden parts of the object afterwards